1. Field of the Invention
The present invention relates to a liquid crystal backlight apparatus, and more specifically, to a backlight apparatus for a personal computer, a liquid crystal monitor, and a liquid crystal TV.
2. Description of the Related Art
FIG. 26 shows a configuration of a Rosen type piezoelectric transformer, which is a typical configuration of a conventional piezoelectric transformer. This piezoelectric transformer enables a very high voltage step-up ratio to be obtained when a load is infinite and enables a voltage step-up ratio to be decreased when a load becomes smaller. Furthermore, the piezoelectric transformer has the following advantages: it can be miniaturized compared with an electromagnetic transformer, is nonflammable, and does not generate noise due to electromagnetic induction. Because of these features, the piezoelectric transformer has been used as a power source for a cold-cathode fluorescent tube.
In FIG. 26, a portion denoted with reference numeral 1 is a low impedance portion that functions as an input portion in the case of the use for stepping-up a voltage. The low impedance portion 1 is provided with polarization PD in the thickness direction of a piezoelectric layer 5, and electrodes 3U and 3D are placed on principal planes in the thickness direction of the piezoelectric layer 5. On the other hand, a portion denoted with reference numeral 2 is a high impedance portion that functions as an output portion in the case of the use for stepping-up a voltage. The high impedance portion 2 is provided with polarization PL in the longitudinal direction of a piezoelectric layer 7, and an electrode 4 is placed on an end face in the longitudinal direction.
Next, an operation of a Rosen type piezoelectric transformer with a conventional configuration shown in FIG. 26 will be described with reference to FIG. 27.
FIG. 27 shows a lumped parameter approximate equivalent circuit in the vicinity of a resonance frequency of a Rosen type piezoelectric transformer. In FIG. 27, Cd1 and Cd2 are input side and output side bound capacitors, respectively; A1 (input side) and A2 (output side) are force factors; m is an equivalent mass, C is an equivalent compliance, and Rm is an equivalent mechanical resistor. In this piezoelectric transformer, the force factor A1 is larger than the force factor A2, and a voltage is stepped-up by two equivalent ideal transformers in FIG. 27. Furthermore, the piezoelectric transformer includes a series resonance circuit composed of the equivalent mass m and the equivalent compliance C. Therefore, particularly in the case where the value of a load resistance is large, an output voltage has a value larger than a ratio of transformation of the transformers.
Furthermore, a cold-cathode fluorescent tube with a cold-cathode configuration, in which an electrode for discharge has no heater, generally is used as a backlight. Because of the cold-cathode configuration, in the cold-cathode fluorescent tube, a discharge start voltage for starting discharge and a discharge keeping voltage for maintaining discharge are both very high. Generally, a cold-cathode fluorescent tube used in a 14-inch liquid crystal display requires about 800 Vrms of a discharge keeping voltage and about 1300 Vrms of a discharge start voltage.
It is considered that a lighting start voltage and a lighting maintenance voltage will be increased due to the enlargement of a liquid crystal display and the increased length of a cold-cathode fluorescent tube.
FIG. 28 shows a block diagram showing an exemplary configuration of a light-emission control apparatus (piezoelectric inverter) of a cold-cathode fluorescent tube using a conventional piezoelectric transformer. In FIG. 28, reference numeral 13 denotes a variable oscillation circuit generating an AC driving signal that drives a piezoelectric transformer 10. An output signal of the variable oscillation circuit 13 generally has a pulse waveform, and has a high-frequency component removed by a wave-shaping circuit 11 to be converted to an AC signal close to a sine wave. An output signal of the wave-shaping circuit 11 is amplified in a voltage to a sufficient level for allowing a driving circuit 12 to drive the piezoelectric transformer 10, and is input to a primary side electrode 3U of the piezoelectric transformer 10. An output voltage stepped-up by the piezoelectric effect of the piezoelectric transformer 10 is taken out from a secondary side electrode 4.
A high voltage output from the secondary side electrode 4 is applied to a series circuit composed of a cold-cathode fluorescent tube 17 and a feedback resistor 18, and an overvoltage protecting circuit 20. In the overvoltage protecting circuit 20, a comparison circuit 15 compares a voltage detected by voltage division resistors 19a and 19b with a set voltage Vref1. An output signal of the comparison circuit 15 is supplied to an oscillation control circuit 14 so as to control the variable oscillation circuit 13 so that the high voltage output from the secondary side electrode 4 of the piezoelectric transformer 10 is prevented from being higher than a voltage determined by the set voltage Vref1. The overvoltage protecting circuit 20 is not operated while the cold-cathode tube fluorescent tube 17 is lighting.
Furthermore, a comparison circuit 16 is supplied with a voltage generated at both ends of the feedback resistor 18 due to a current flowing through the series circuit composed of the cold-cathode fluorescent tube 17 and the feedback resistor 18. The comparison circuit 16 compares a feedback voltage with a set voltage Vref2, and sends a signal to the oscillation control circuit 14 so that a substantially constant current flows through the cold-cathode fluorescent tube 17. The oscillation control circuit 14 applies an output signal to the variable oscillation circuit 13 so that the variable oscillation circuit 13 oscillates at a frequency in accordance with output signals from the comparison circuits 15 and 16. The comparison circuit 16 is not operated before the commencement of lighting of the cold-cathode fluorescent tube 17.
Thus, the cold-cathode fluorescent tube 17 lights up stably. In the case where the cold-cathode fluorescent tube 17 is driven by a method as described here, even though a resonance frequency is varied depending upon the temperature, a driving frequency follows the resonance frequency automatically.
Because of the above configuration of a piezoelectric inverter, a current flowing through the cold-cathode fluorescent tube can be controlled to be constant. Furthermore, the emission brightness of the cold-cathode fluorescent tube is controlled to be constant by setting an output current and an output voltage of the piezoelectric transformer to be constant or by detecting a current flowing through a reflector.
In the conventional piezoelectric inverter as described above, in the case where the cold-cathode fluorescent tube is lit up, a resistor is connected to the side of the cold-cathode fluorescent tube close to the ground, a voltage corresponding to a tube current is detected, and a driving frequency is varied, whereby the emission brightness of the cold-cathode fluorescent tube is controlled. As a result, there is a possibility that driving is performed apart from a resonance frequency (herein, this refers to a frequency at which a voltage step-up ratio becomes maximum), which causes a decrease in efficiency or the like.
Furthermore, in order to solve the above-mentioned problem, it is required to add a circuit for regulating a power supply voltage, which hinders the miniaturization of an inverter circuit.
For the above reasons, the following problems need to be solved in terms of the miniaturization and decreased thickness of liquid crystal equipment.
(1) Reduction in the number of components and saving of space.
(2) Reduction in driving waveform distortion with respect to a harmonic component.
(3) Reduction in distortion caused by a large amplitude operation at the commencement of lighting.
(4) High-efficiency operation during lighting.
Regarding (1), a conventional piezoelectric inverter is composed of analog circuits, so that the number of components is reduced by an analog IC. However, when a driving circuit is composed of one chip, a small package cannot be used due to the limitation of the number of pins; therefore, its effect is small.
A method for composing a driving circuit with a digital LSI of a liquid crystal controller as one chip is considered. In this case, it is desirable that digital control is performed in the piezoelectric inverter.
However, in order to digitize the piezoelectric inverter, a clock with a high frequency is required so as to ensure the frequency precision required for controlling the piezoelectric transformer. This necessitates a high frequency resolution in a system controlling an output power with a driving frequency of the piezoelectric transformer as in the prior art. Because of this, an IC operated at a high clock frequency as described above is required.
Regarding (2), in the conventional analog circuit, driving is conducted in such a manner that a signal with a rectangular waveform of a small signal is amplified in an electric power by a switching element, a low-pass filter is composed of an inductor L and a primary side capacitor C01 of a piezoelectric transformer, and a substantially sine wave is input to the piezoelectric transformer. The low-pass filter prevents a driving waveform containing a large amount of harmonic signals from being input. However, harmonic components cannot be removed completely, and due to the sweep of a frequency, the harmonic of a driving frequency is matched with a vibration mode of the piezoelectric transformer during light-adjusment control of a cold-cathode fluorescent tube. Consequently, waveform distortion of an input/output voltage and harmonic distortion of a piezoelectric transformer occur.
Regarding (3), for the purpose of the miniaturization and decrease in thickness of an inverter, the miniaturization and decrease in thickness of a piezoelectric transformer are required. However, a high voltage is required at the commencement of lighting of a cold-cathode fluorescent tube, and a large amplitude operation also is required in the piezoelectric transformer so as to output the high voltage. Due to the problem of distortion caused by the large amplitude operation, there is a limitation to the miniaturization of the piezoelectric transformer. For example, JP 2001-136749 A discloses a piezoelectric inverter driving apparatus for controlling the frequency of a driving signal of a piezoelectric transformer at the commencement of lighting (during startup) of a cold-cathode fluorescent tube and controlling the phase difference of a driving signal during steady lighting. Such an apparatus controls a frequency during startup. Therefore, in the case where a driving frequency is varied with the same step width as that of DF0′, DF1′, and DF2′ as represented by TP101′ in FIG. 9, an output voltage Vout of the piezoelectric transformer approaches a peak of TP101′, and a change in the output voltage Vout with respect to one step width of frequency control becomes very large, i.e., ΔVout1 and ΔVout2 (for example, several tens of times change amount of a voltage step-up ratio). This causes a large amplitude operation, making it difficult to miniaturize the piezoelectric transformer.
Regarding (4), the efficiency of a piezoelectric transformer becomes maximum in the vicinity of a resonance frequency due to its operation principle; however, the efficiency is decreased with distance from a resonance frequency. It is effective that the piezoelectric transformer is driven at a frequency closer to a resonance frequency as much as possible. For example, JP 10(1998)-178784 A proposes a method for performing PWM control with respect to an input voltage so that the piezoelectric transformer can be driven in the vicinity of a resonance frequency. However, in the case where waveform processing is performed with an input voltage, as described in (2), waveform distortion occurs due to a harmonic component, and it becomes difficult to reduce waveform distortion during steady operation.
Furthermore, JP 7(1995)-39144 A and JP 7(1995)-59338 A propose methods for controlling an input voltage of a piezoelectric transformer, using two switching elements. However, according to the methods disclosed therein, a voltage applied to the piezoelectric transformer is a half of an input voltage in terms of an AC voltage, so that it is required to step-up a voltage.